The present invention relates to methods of treating sewage sludge having a high initial water content to simplify problems of handling and disposal.
Raw or activated sludge from typical industrial and municipal sewage treatment plants usually contains at least 75% by weight of water. The sludge is a gelatinous, thixotropic substance which makes pumping, or handling by other conventional means, extremely difficult. This complicates the problem of transportation of the sludge to disposal sites.
The disposal itself may present problems of both environmental pollution and of energy consumption. Although both land and water disposal of unprocessed sludge are widely practised as the only viable alternatives presently available, it is generally recognized that they cannot continue indefinitely. Disposal by burning is extremely inefficient due to the enormous quantity of energy required to drive off the water contained by the sludge.
From the foregoing, it is apparent that the major problems associated with both physical handling and disposal of sewage sludges would be minimized by extracting a significant portion of the water initially contained thereby. However, dewatering of the sludge presents further problems, due to the nature of the substance. That is, dewatering by conventional, economical means such as filtration or normal evaporation are impractical since the water is entrapped by the molecular structure of the sludge and the tendency to form a crust which further inhibits air drying.
Chemical treatment of sewage sludges has been advanced as a dewatering method, for example, in U.S. Pat. No. 3,772,191 of Thorn. According to this method, the sludge is first treated with a concentrated mineral acid to reduce the water content to the vicinity of 80%, and then mixed with calcium hydroxide to facilitate further dewatering in a filter press. Other processes which involve contacting sludge solids with other chemicals include those set forth in U.S. Pat. Nos. 3,050,383 of Wilson and 3,758,287 of Scheel. However, these patents are directed primarily to treating or producing other products and utilize sludge which has been previously dewatered by some means to a water content much lower than that of typical fresh sludge.
It is a principal object of the present invention to provide a process for dewatering previously unprocessed sewage sludge to a manageable level by chemical treatment which is efficient and economical, as well as ecologically acceptable.
A further object is to provide a method of processing fresh sewage sludge to significantly reduce the water content thereof in an easy and economical manner to a level facilitating further dewatering by other processes.
Another object is to provide a method of dewatering sewage sludge which allows economical recovery and use of essentially all constituents of the process.
A still further object is to provide a process of chemically treating sewage sludge which yields useful end products with no intermediate or by-products which create environmental pollution problems.
Other products will in part be obvious and will in part appear hereinafter.